Dire shortages of two critical cancer drugs—shortfalls that have threatened the lives and care of thousands of patients—should be resolved within weeks, federal drug officials said. In the case of Doxil, the F.D.A. has decided to allow temporary shipments from India of Lipodox, which is similar to Doxil and is made by Sun Pharma Global. And the pharmaceutical company Hospira is rushing 31,000 vials—enough to last the entire nation a month—of preservative-free methotrexate from its plant in Australia to the United States. Hospitals began receiving the drug, which is vital in the treatment of a common form of childhood leukemia, on Tuesday.